Consequences of Fetal Irradiation on Follicle Histogenesis and Early Follicle Development in Rat Ovaries1

Loss of Calponin 2 causes premature ovarian insufficiency in mice

BACKGROUND

Premature ovarian insufficiency (POI) is a condition defined as women developing menopause before 40 years old. These patients display low ovarian reserve at young age and difficulties to conceive even with assisted reproductive technology. The pathogenesis of ovarian insufficiency is not fully understood. Genetic factors may underlie most of the cases. Actin cytoskeleton plays a pivotal role in ovarian folliculogenesis. Calponin 2 encoded by the Cnn2 gene is an actin associated protein that regulates motility and mechanical signaling related cellular functions.

RESULTS

The present study compared breeding of age-matched calponin 2 knockout (Cnn2-KO) and wild type (WT) mice and found that Cnn2-KO mothers had significantly smaller litter sizes. Ovaries from 4 weeks old Cnn2-KO mice showed significantly lower numbers of total ovarian follicles than WT control with the presence of multi-oocyte follicles. Cnn2-KO mice also showed age-progressive earlier depletion of ovarian follicles. Cnn2 expression is detected in the cumulus cells of the ovarian follicles of WT mice and colocalizes with actin stress fiber, tropomyosin and myosin II in primary cultures of cumulus cells.

CONCLUSIONS

The findings demonstrate that the loss of calponin 2 impairs ovarian folliculogenesis with premature depletion of ovarian follicles. The role of calponin 2 in ovarian granulosa cells suggests a molecular target for further investigations on the pathogenesis of POI and for therapeutic development.

The neonatal southern white rhinoceros ovary contains oogonia in germ cell nests

The northern white rhinoceros is functionally extinct with only two females left. Establishing methods to culture ovarian tissues, follicles, and oocytes to generate eggs will support conservation efforts using in vitro embryo production. To the best of our knowledge, this is the first description of the structure and molecular signature of any rhinoceros, more specifically, we describe the neonatal and adult southern white rhinoceros (Ceratotherium simum simum) ovary; the closest relation of the northern white rhinoceros. Interestingly, all ovaries contain follicles despite advanced age. Analysis of the neonate reveals a population of cells molecularly characterised as mitotically active, pluripotent with germ cell properties. These results indicate that unusually, the neonatal ovary still contains oogonia in germ cell nests at birth, providing an opportunity for fertility preservation. Therefore, utilising ovaries from stillborn and adult rhinoceros can provide cells for advanced assisted reproductive technologies and investigating the neonatal ovaries of other endangered species is crucial for conservation.

In Utero and Lactational Exposure to Flame Retardants Disrupts Rat Ovarian Follicular Development and Advances Puberty

Brominated flame retardants (BFRs), including polybrominated diphenyl ethers and hexabromocyclododecane, leach out from consumer products into the environment. Exposure to BFRs has been associated with effects on endocrine homeostasis. To test the hypothesis that in utero and lactational exposure to BFRs may affect the reproductive system of female offspring, adult female Sprague Dawley rats were fed diets formulated to deliver nominal doses (0, 0.06, 20, or 60 mg/kg/day) of a BFR dietary mixture mimicking the relative congener levels in house dust from prior to mating until weaning. Vaginal opening and the day of first estrus occurred at a significantly earlier age among offspring from the 20 mg/kg/day BFR group, indicating that the onset of puberty was advanced. Histological analysis of ovaries from postnatal day 46 offspring revealed an increase in the incidence of abnormal follicles. A toxicogenomic analysis of ovarian gene expression identified upstream regulators, including HIF1A, CREB1, EGF, the β-estradiol, and PPARA pathways, predicted to be downregulated in the 20 or 60 mg/kg/day group and to contribute to the gene expression patterns observed. Thus, perinatal exposure to BFRs dysregulated ovarian folliculogenesis and signaling pathways that are fundamental for ovarian function in the adult.

Oestrogenic action of neonatal tamoxifen on the hypothalamus and reproductive system in female mice

Tamoxifen, a selective oestrogen receptor modulator, is widely used for both the treatment and prevention of breast cancer in women; however, it is known to have adverse effects in the female reproductive system. Growing evidence suggests that oestrogen-sensitive neuron populations of the anteroventral periventricular (AVPV) nucleus and arcuate (ARC) nucleus, especially kisspeptin neurons, play a pivotal role in the timing of puberty onset and reproductive function. The aim of the present study was to evaluate whether neonatal exposure to tamoxifen affects oestrogenic actions in the brain and reproductive function in mice. On 1 to 5 postnatal days, female pups were injected subcutaneously with sesame oil (sham), oestradiol benzoate (EB; 20 µg kg-1), tamoxifen (0.4 mg kg-1) or EB+tamoxifen. Control mice received no treatment. Mice in the EB, tamoxifen and tamoxifen+EB groups exhibited advanced vaginal opening, disrupted oestrous cycles and a decreased follicular pool. Conversely, in these groups, there was a reduction in kisspeptin (Kiss1) mRNA expression, the neuronal density of AVPV and ARC nuclei and LH and oestradiol concentrations in the serum. The results of the present study confirm oestrogenic actions of tamoxifen in the brain and reproductive system. In addition, we show, for the first time, that tamoxifen has oestrogenic effects on the oestrogen-sensitive hypothalamic AVPV and ARC nuclei controlling the reproductive axis in female mice.

Effects of deletion of the transcription factor Nrf2 and benzo [a]pyrene treatment on ovarian follicles and ovarian surface epithelial cells in mice

Polycyclic aromatic hydrocarbons, like benzo[a]pyrene (BaP), are ubiquitous environmental pollutants and potent ovarian toxicants. The transcription factor NRF2 is an important regulator of the cellular response to electrophilic toxicants like BaP and to oxidative stress. NRF2 regulates transcription of genes involved in the detoxification of reactive metabolites of BaP and reactive oxygen species. We therefore hypothesized that Nrf2-/- mice have accelerated ovarian aging and increased sensitivity to the ovarian toxicity of BaP. A single injection of BaP dose-dependently depleted ovarian follicles in Nrf2+/+ and Nrf2-/- mice, but the effects of BaP were not enhanced in the absence of Nrf2. Similarly, Nrf2-/- mice did not have increased ovarian BaP DNA adduct formation compared to Nrf2+/+ mice. Ovarian follicle numbers did not differ between peripubertal Nrf2-/- and Nrf2+/+ mice, but by middle age, Nrf2-/- mice had significantly fewer primordial follicles than Nrf2+/+ mice, consistent with accelerated ovarian aging.

Generation of multi-oocyte follicles in the peripubertal rat ovary: link to the invasive capacity of granulosa cells?

OBJECTIVE

To assess the presence and numbers of multi-oocyte follicles (MOFs) in the rat ovary at different stages of postnatal maturation.

DESIGN

Animal (rat) study.

SETTING

Research laboratory.

ANIMAL(S)

Female Wistar rats.

INTERVENTION(S)

Histologic/morphometric analyses in ovaries from infantile, juvenile, pubertal, and adult female rats.

MAIN OUTCOME MEASURE(S)

Numbers and characteristics of MOFs in rat ovaries at different stages of postnatal maturation.

RESULT(S)

Female rats displayed low numbers (<5/ovary) of MOFs in the infantile period (postnatal day [PND] 15). The occurrence of MOFs increased sharply by PND-21 and remained at high values (>15/ovary) up to PND-60, to decline thereafter by PND-90. The presence of irregularly shaped and connected adjacent follicles, together with the identification of ruptures at the follicle surface and the occasional invasion of the ovarian stroma by granulosa cells, strongly suggests that the majority of MOFs in peripubertal rats are generated by fusion of adjacent growing follicles.

CONCLUSION(S)

A new mechanism for the generation of MOFs linked to the potential invasive capacity of granulosa cells is proposed. The basis for the upsurge in the generation of MOFs during the peripubertal period and whether, as predictable, this phenomenon is applicable to other mammalian species warrant further investigation.

Systemic compensatory response to neonatal estradiol exposure does not prevent depletion of the oocyte pool in the rat

The formation of ovarian follicles is a finely tuned process that takes place within a narrow time-window in rodents. Multiple factors and pathways have been proposed to contribute to the mechanisms triggering this process but the role of endocrine factors, especially estrogens, remains elusive. It is currently hypothesized that removal from the maternal hormonal environment permits follicle formation at birth. However, experimentally-induced maintenance of high 17β-estradiol (E2) levels leads to subtle, distinct, immediate effects on follicle formation and oocyte survival depending on the species and dose. In this study, we examined the immediate effects of neonatal E2 exposure from post-natal day (PND) 0 to PND2 on the whole organism and on ovarian follicle formation in rats. Measurements of plasma E2, estrone and their sulfate conjugates after E2 exposure showed that neonatal female rats rapidly acquire the capability to metabolize and clear excessive E2 levels. Concomitant modifications to the mRNA content of genes encoding selected E2 metabolism enzymes in the liver and the ovary in response to E2 exposure indicate that E2 may modify the neonatal maturation of these organs. In the liver, E2 treatment was associated with lower acquisition of the capability to metabolize E2. In the ovary, E2 depleted the oocyte pool in a dose dependent manner by PND3. In 10 µg/day E2-treated ovaries, apoptotic oocytes were observed in newly formed follicles in addition to areas of ovarian cord remodeling. At PND6, follicles without any visible oocyte were present and multi-oocyte follicles were not observed. Our study reveals a major species-difference. Indeed, neonatal exposure to E2 depletes the oocyte pool in the rat ovary, whereas in the mouse it is well known to increase oocyte survival.

The primordial follicle reserve is not renewed after chemical or γ-irradiation mediated depletion

Reports indicate that germ-line stem cells present in adult mice can rapidly generate new oocytes and contribute to the primordial follicle reserve following conditions of ovotoxic stress. We further investigated the hypothesis that adult mice have the capacity to generate new oocytes by monitoring primordial follicle numbers throughout postnatal life and following depletion of the primordial follicle reserve by exposure to doxorubicin (DXR), trichostatin A (TSA), or whole-body γ-irradiation. We show that primordial follicle number remains stable in adult C57BL/6 mice between the ages of 25 and 100 days. However, within 2 days of treatment with DXR or TSA, primordial follicle numbers had declined to 65 and 51% respectively (P<0.05-0.01 when compared to untreated controls), with no restoration of follicle numbers evident after 7 days for either treatment. Furthermore, ovaries from mice subjected to sterilizing doses of γ-irradiation (0.45 or 4.5 Gy) revealed complete ablation of all primordial follicles 5 days after treatment, with no indication of follicular renewal. We conclude that neo-folliculogenesis does not occur following chemical or γ-irradiation mediated depletion of the primordial follicle reserve.

[Reconsidering the roles of female germ cells in ovarian development and folliculogenesis]

The production of fertilizable ova is the consequence of multiple events that start as soon as ovarian development and culminate at the time of ovulation. Throughout their development, germ cells are associated with companion somatic cells, which ensure germ cell survival, growth and maturation. Data obtained in vitro and in vivo on several animal models of germ cell depletion have led to uncover the many roles of germ cells on both ovarian development and folliculogenesis. During ovarian development, germ cells become progressively enclosed within epithelial structures called "ovigerous cords" constituted by pregranulosa cells, lined by a basement membrane. At the end of ovarian development, ovigerous cords fragment into primordial follicles, which are epithelial units constituted by an oocyte surrounded by a single layer of granulosa cells. Germ cells are necessary for the fragmentation of ovigerous cords into follicles, since in their absence, no follicle will form. Germ cells also ensure the differentiation of the ovarian somatic lineage, and they may inhibit the testis-differentiating pathway by preventing the conversion of pregranulosa cells into Sertoli cells, their counterpart in the testis. Regularly, primordial follicles are recruited into the growing follicle pool and initiate their growth. They develop through primary, preantral, antral and preovulatory stages before being ovulated. Interestingly, the action of the oocyte on companion somatic cells tightly depends on the follicular stage. In primordial follicles, the oocyte prevents the transdifferentiation of granulosa cells into cells resembling Sertoli cells. By contrast, as soon as the follicle enters growth, the oocyte regulates the functional differentiation of granulosa cells and at the latest stages, it prevents their premature maturation into luteal cells. Overall, these data demonstrate that the female germ cell act on companion somatic cells to regulate ovarian development and folliculogenesis, thereby actively supporting its own maturation.

An immunohistochemical study of ovarian follicle histogenesis in the early post-hatch Japanese quail (Coturnix coturnix japonica)

The early post-hatch development of immunoreactivity to vimentin, desmin, smooth muscle actin (SMA) and laminin, in relation to follicle histogenesis, was described in this study. Ovigerous cords in day old quails contained pre-granulosa cells and oocytes. Pre-granulosa cells at the cortico-medullary junction were vimentin immunopositive. A laminin immunopositive basement membrane and desmin immunopositive mesenchymal cells lined the ovigerous cords. Ovigerous cords in 3-day-old quails contained developing primordial follicles, the vimentin immunopositive pre-granulosa cells of which were partially encircled by a basement membrane and desmin immunopositive mesenchymal cells. In 5- to 7-day-old quails, ovigerous cords formed an outer cortical region, while primordial follicles formed the inner cortical region. Early pre-vitellogenic follicles were present in 9- to 13-day-old quails. Underlying the granulosa cells of these follicles was a laminin immunopositive basement membrane and a layer of desmin immunopositive thecal cells. Early and late pre-vitellogenic follicles dominated the ovary in 15- to 17-day-old quails. The thecal layer in these follicles was desmin immunopositive, but SMA immunonegative. The results of the study have shown that the process of primordial follicle development in the Japanese quail is similar to that reported in mammals. The study suggests that in the quail pre-granulosa cells originate predominantly from the medulla. The study has shown that, in the Japanese quail, thecal cells are derived from desmin immunopositive mesenchymal cells lining the ovigerous cords.

Prevention of premature ovarian failure and osteoporosis induced by irradiation using allogeneic ovarian/bone marrow transplantation

BACKGROUND

Two side effects of irradiation are premature ovarian failure (POF) and osteoporosis, both of which are concerns not only clinically, for patients, but also experimentally, for animals. We examine whether bone marrow transplantation (BMT) can correct the POF induced by radiation and also address whether allogeneic ovarian transplantation (OT) can modulate the adverse effects of radiotherapy.

METHODS

Eight-week-old female C57BL/6 mice were lethally irradiated with 6 Gy x2, and then injected with allogeneic bone marrow cells into their bone marrow cavity using our previously described intrabone marrow (IBM)-BMT technique. Allogeneic ovaries were simultaneously transplanted under the renal capsules of the mice.

RESULTS

Three months after the transplantation, we noted that hematopoietic and lymphoid cells had been successfully reconstituted. The ovaries transplanted under the renal capsules demonstrated signs of development with a large number of differentiating follicles at different stages of development. Importantly, the total bone mineral density of the tibia in the "IBM-BMT+OT" (BMT/OT) group remained normal. However, the reproductive function of the recipient mice was not restored, despite the presence of many immature oocytes in the host ovaries in the BMT/OT group. In the BMT group, no oocytes were found in the host ovaries.

CONCLUSIONS

These findings suggest that IBM-BMT with ovarian allografts can be advantageous for young women with POF and osteopenia or osteoporosis that is due to chemotherapy and radiotherapy for malignant diseases.

Suppression of Notch signaling in the neonatal mouse ovary decreases primordial follicle formation

Notch signaling directs cell fate during embryogenesis by influencing cell proliferation, differentiation, and apoptosis. Notch genes are expressed in the adult mouse ovary, and roles for Notch in regulating folliculogenesis are beginning to emerge from mouse genetic models. We investigated how Notch signaling might influence the formation of primordial follicles. Follicle assembly takes place when germ cell syncytia within the ovary break down and germ cells are encapsulated by pregranulosa cells. In the mouse, this occurs during the first 4-5 d of postnatal life. The expression of Notch family genes in the neonatal mouse ovary was determined through RT-PCR measurements. Jagged1, Notch2, and Hes1 transcripts were the most abundantly expressed ligand, receptor, and target gene, respectively. Jagged1 and Hey2 mRNAs were up-regulated over the period of follicle formation. Localization studies demonstrated that JAGGED1 is expressed in germ cells prior to follicle assembly and in the oocytes of primordial follicles. Pregranulosa cells that surround germ cell nests express HES1. In addition, pregranulosa cells of primordial follicles expressed NOTCH2 and Hey2 mRNA. We used an ex vivo ovary culture system to assess the requirement for Notch signaling during early follicle development. Newborn ovaries cultured in the presence of gamma-secretase inhibitors, compounds that attenuate Notch signaling, had a marked reduction in primordial follicles compared with vehicle-treated ovaries, and there was a corresponding increase in germ cells that remained within nests. These data support a functional role for Notch signaling in regulating primordial follicle formation.

Gonad morphogenesis in vertebrates: divergent means to a convergent end

A critical element of successful sexual reproduction is the generation of sexually dimorphic adult reproductive organs, the testis and ovary, which produce functional gametes. Examination of different vertebrate species shows that the adult gonad is remarkably similar in its morphology across different phylogenetic classes. Surprisingly, however, the cellular and molecular programs employed to create similar organs are not evolutionarily conserved. We highlight the mechanisms used by different vertebrate model systems to generate the somatic architecture necessary to support gametogenesis. In addition, we examine the different vertebrate patterns of germ cell migration from their site of origin to colonize the gonad and highlight their roles in sex-specific morphogenesis. We also discuss the plasticity of the adult gonad and consider how different genetic and environmental conditions can induce transitions between testis and ovary morphology.

Development of primordial follicles in the hamster: role of estradiol-17beta

The role of E2 on primordial follicle formation was examined by treating neonatal hamsters with 1 or 2 microg estradiol cypionate (ECP) at age postnatal d 1 (P1) and P4 or by in vitro culture of embryonic d 15 (E15) ovaries with 1, 5, or 10 ng/ml estradiol-17beta (E2). The specificity of E2 action was examined by ICI 182,780. One microgram of ECP maintained serum levels of E2 within the physiological range, significantly reduced apoptosis, and stimulated the formation and development of primordial follicles. In contrast, 2 microg ECP increased serum E2 levels to 400 pg/ml and had significantly less influence on primordial follicle formation. In vivo, ICI 182,780 significantly increased apoptosis and caused a modest reduction in primordial follicle formation. The formation and development of primordial follicles in vitro increased markedly with 1 ng/ml E2, and the effect was blocked by ICI 182,780. Higher doses of E2 had no effect on primordial follicle formation but significantly up-regulated apoptosis, which was blocked by ICI 182,780. CYP19A1 mRNA expression occurred by E13 and increased with the formation of primordial follicles. P4 ovaries synthesized E2 from testosterone, which increased further by FSH. Both testosterone and FSH maintained ovarian CYP19A1 mRNA, but FSH up-regulated the expression. These results suggest that neonatal hamster ovaries produce E2 under FSH control and that E2 action is essential for the survival and differentiation of somatic cells and the oocytes leading to the formation and development of primordial follicles. This supportive action of E2 is lost when hormone levels increase above a threshold.

From primordial germ cell to primordial follicle: mammalian female germ cell development

In mammals, the final number of oocytes available for reproduction of the next generation is defined at birth. Establishment of this oocyte pool is essential for fertility. Mammalian primordial germ cells form and migrate to the gonad during embryonic development. After arriving at the gonad, the germ cells are called oogonia and develop in clusters of cells called germ line cysts or oocyte nests. Subsequently, the oogonia enter meiosis and become oocytes. The oocyte nests break apart into individual cells and become packaged into primordial follicles. During this time, only a subset of oocytes ultimately survive and the remaining immature eggs die by programmed cell death. This phase of oocyte differentiation is poorly understood but molecules and mechanisms that regulate oocyte development are beginning to be identified. This review focuses on these early stages of female germ cell development.